Film scanners are used to generate video signals or data from photographic film originals. Three basic methods are known in this case: one method uses an areal array sensor (electronic camera) onto which the image is projected and converted into electrical signals. A further method uses a so-called “flying spot” scanner, in which an electron beam is used to write a raster onto the surface of a cathode ray tube. The said raster is imaged onto the film to be scanned and is converted into electrical signals by means of photomultiplier tubes or semiconductors, such as e.g. photodiodes.
A third method uses CCD linear array sensors which supply a serial pixel data stream at the output. In this case, the film to be scanned moves continuously between an illumination device and the CCD sensors, the film image being focussed onto the CCDs by means of an imaging optical arrangement. Three CCDs are often used for the colour separations red, green and blue, the spectral splitting of the colour separations being performed by means of a dichroic light splitter (U.S. Pat. No. 4,205,337).
There are limits, however, with regard to the achievable speed at which such CCD sensors can be read out with high signal quality. The said limits depend e.g. on the semiconductor technology used and on the available clock drivers for the charge shifting circuit. A high readout speed is required, however, if the resolution and thus the number of pixels are increased without slowing down the scan rate (scanning speed in images per second).
In order to simultaneously satisfy the requirements of a high resolution and of fast scanning (e.g. 30 images per second), the sensors are embodied with a plurality of shift register channels and output stages (channel multiplex). One known film scanner uses e.g. CCDs which have four shift registers and four output stages. The way in which all the pixels are divided between a plurality of channels depends on the CCD architecture. Examples are sensors with four separate image segments or divisions into even-numbered and odd-numbered pixels. The different channels are combined again to form a total image in the downstream signal processing. In a scanner according to this method, the first signal processing stages, usually up to the analogue-to-digital conversion, are therefore embodied in a channel multiplex.
Stringent requirements are made as to the quality of the scanned images in the area of post-processing (film post-processing for e.g. cinema films, advertising). The aim is to convert the high contrast range of a photographic negative film material, ranging over a plurality of focal apertures, into a digital copy as close as possible to the gradation of the film. In this case, the channel multiplex described in the scanner is disadvantageous since even very small differences in the behaviour of the individual channels can lead to visible disturbances in the image. Known methods correct the black value of the individual channels by pixel clamping (correlated double sampling) and line clamping, while the white value is effected by adjustment of the gain of individual channels and a so-called FPN correction (FPN: “Fixed Pattern Noise”). This FPN correction eliminates level errors of individual pixels of the illuminated CCDs by determination of the errors and subsequent correction in a multiplier. Two points of the transfer characteristic curve—black value and white value—are thus corrected with sufficient accuracy. However, deviations from the ideal linear transfer characteristic curve “output voltage as a function of the quantity of light” between these points are not detected, and thus lead to errors. These errors then become particularly visible if the channels are separated on the sensor into different, adjacent image segments.
Taking this as a departure point, it is an object of the present invention to provide an arrangement for generating electrical image signals from an image original in which differences in the transfer functions of different channels are corrected.